Phospholipids

They are built out of glycerol, phosphate, 2 fatty acids and one alcohol.

Six different alcohols can be attached to one end of the glycerol, plus one diglyceride makes seven major phospholipids

Some have specific functions

Phosphatidic acid (PA)

All seven phospholipids can fit in membrane

If we make pH 7.2, charge would be -1.5

So we would have large negative charge density

This is important because many proteins need negative environment to work.

This is an example of a specific function of a phospholipid.

A typical membrane has 1500 different lipids.

Specific functions of phosphatidic acid

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Phophatidyl Ethanolamine (PE)

When in pH 7, we have slight negative charge because the amine will be almost a full positive charge while the OH group on the phosphate group will have a full -1 charge.

Egg yolk has lots of phophatidyl choline and cholesterol

Phophatidyl ethanolamine has a small head area, so when lots of it is side by side it forms a curve.

This is called nom lameller phase.

This phase makes hydration hard (the water molecules have a hard time lining up to the surface) so there is a little dry spot on the surface of the membrane where there is much phosphatidyl ethanolamine: a place of weakness.

Phosphatidyl choline (PC)

This molecule is just like PE only methylated 3 times via s-adenosyl methionine.

We have more PC than any other phospholipid

PC has a big head, is easily hydrated, and is can-shaped.

Because of these features this phospholipid forms membranes perfectly.

The old name of PC is lesophin.

At biological pH (7.0), the OH group on the phosphate has a formal -1 charge and the amine group has a formal +1 charge, so there is a formal charge distribution over the head of the phospholipid.

Phosphotidyl Serine (PS)

This phospholipid has the amino acid serine on it.

At biological pH, the OH group on the phosphate group has a -1 charge (pKa 2.1), the NH3 group has a +1 charge (pKa 9.1) and the carboxyl group will have a -1 charge (pKa 2.1) thus having a net -1 charge.

This phospholipid provides a negative density at biological pH.

At pH 2.1: -0.5, -0.5, +1 = 0. When acitic, these have not charge, think of stomach lining: cells on wall of the stomach will have phospholipids with differing properties on the exterior layer of the bilipid layer verses those of the inner layer.

Many proteins require phosphotidyl Serine.

Less than 10% of our phospholipid material is phosphotidyl serine.

All of our PS is on the interior layer of the bilipid layer. We have flipases to put them on inside whenever they happen to get to the outside layer.

PS on the outside can trigger apoptosis

PS on the outside is seen in aging and disease states.

Phosphatidyl Inositol (PI)

Inositol is a sugar we aren't good at digesting, so it is often put in sugar-free candy because we don't digest it.

It is not a complete sugar.

None of the OH on this ring are dissociable at reasonable pH levels.

Net charge is -1 at 7.2pH because of the OH group on the phosphate group (with pKa 2.1)

This is the best example of phospholipids responsible for lipid diversity.

There are unique functions for phophatidyl Inositol

Can be an anchor in membrane for classes of proteins

A lipid anchor in the membrane.

Example: alcoline phophatase -> a protein that anchors on GPI

So these kinds of proteins are called GPI anchored proteins.

GPI = Glycosil phosphatidyl inositol

Hormonal Role

PI + Phospholipase-c -> diacylglicerol (amphipathic and membrane bound) and the phosphate connected to the inositol (very water soluble, binds with signal proteins).

Both these products are activators to mechanisms.

Phosphatidyl glycerol (PG)

At pH 7 there is a -1 net charge because of the OH group with pKa 2.1 on the phosphate group.

These are found in plants and prokaryotes.

There are two optically active carbons on this phospholipid.

Cardiolipid (CA)

The old name for this phospholipid is diphosphatidyl glycerol

At pH 7, there is a net -2 charge from the OH on each of the two phosphate groups (pKa 2.1); each has a -1 charge.

This phospholipid has 4 acyl chains and two formal charges, therefore it pretty much looks like two phospholipids stuck together.

This phospholipid is found in the mitochondrial inner membrane, therefore it probably has something to do with the electron transport chain and oxidative phosphorylation (the ability to make ATP).